The method of claim 1, wherein R6, R7 and R8 are independently the same or different and are a C1-6 alkyl group, a phenyl group or a --(CH2)N R9 group, wherein N is an integer within the range of 1 through 6 and R9 is a hydroxy group, alkoxy group an amino group, an alkylamino group a dialkylamino group, a halogen atom, a cyano group or a nitro group.